1. Field of the Invention
The present invention relates to an optical disk (disc) apparatus, and more particularly to an improved tracking control system provided with 3-beam optical pickup.
2. Description of the Prior Art
Configuration and operation of conventional optical disk apparatus will be explained with reference to FIG. 1 and FIG. 2.
A plurality of information tracks are formed on a compact disk (CD), or on a CD-ROM (read only memory) disk spirally. To read the information tracks successively, an optical pickup from which a position error signal is produced, has to be driven to trace an intended track. By counting the produced position error signal, an optical beam of the optical pick up is controlled to reach the intended track, thereafter the optical beam is kept to trace the intended track by a tracking control servo system which operates so as to keep the position error signal zero. Then, the information recorded in the track is read out by the optical pickup. Thus, keeping the position error signal to be zero is an important function for an information memory disk apparatus.
For securing good tracing ability of the optical pickup, the optical disk apparatus is required to have a wide frequency reproducing range, and a large signal to noise ratio. To realize the above, the optical disk apparatus should have such functions as follows.
1) The tracking control servo system is not disturbed by low frequency components of a reproduced information signal. PA1 2) To be resistant to a disturbance of signal recorded in adjacent tracks (crosstalk). PA1 3) To be resistant to an effect of performance degradation of the optical disk apparatus as it ages, that is, a positional displacement of photodetectors for tracking by a change of ambient temperature, and a displacement of beam spot position on a disk due to deformation of associated optical system. PA1 1) The tracking error signal is subject to a low frequency crosstalk of pit information. PA1 2) The tracking system is vulnerable to a crosstalk from adjacent tracks as the track pitch is becoming small. PA1 3) Occurrence of unintended track jam or skip. PA1 4) A performance degradation as it ages. PA1 5) Positional displacement of the photodetectors by the change of the ambient temperature. PA1 6) A tolerance of inclination of the disk surface is small, which leads to that a tolerance of defocus of the laser beam is small. PA1 a light source emitting a light beam and irradiating the disk surface, PA1 an objective lens focusing the main beam and the first and second sub beams on the disk surface, PA1 an actuator for displacing the objective lens so as to move the main beam and the sub beams in a radial direction of the optical disk, PA1 a 4-division optical detector divided into 4 sections for sensing the main beam reflected from the disk surface and outputting 4 divisional electric signals of first, second, third and fourth divisional signals, PA1 first and second optical detectors for sensing first and second sub beams respectively and outputting first and second detected signals respectively, PA1 a first subtracter for subtracting the second detected signal of the second sub-beam from the first detected signal of the first sub-beam and outputting a first subtracted signal, PA1 a low-pass filter (LPF) for passing low frequency component of the first subtracted signal, where frequencies of the low frequency components are lower than a predetermined value, PA1 a first adder for adding the first divisional signal to the fourth divisional signal and outputting a first added signal, PA1 a second adder for adding the second divisional signal and the third divisional signal and outputting a second added signal, PA1 a second subtracter for subtracting the second added signal from the first added signal and outputting a second subtracted signal, PA1 a high-pass filter (HPF) for passing high frequency components of the second subtracted signal, where frequencies of the high frequency components are higher than a predetermined value, PA1 a third adder for adding outputs of the LPF and the HPF together and outputting a tracking error signal, and a tracking drive circuit for controlling the actuator displacing the objective lens. PA1 a light source emitting a light beam and irradiating the disk surface, PA1 an objective lens focusing the main beam and the first and the second sub-beams on the disk surface, PA1 an actuator for displacing the objective lens so as to move the main beam and the sub beams in a radial direction of the optical disk, PA1 a 2-division optical detector divided into 2 sections for sensing the main beam reflected from the disk surface and outputting 2 divisional electric signals of first and second divisional signals, PA1 first and second optical detectors for sensing first and second sub-beams respectively and outputting first and second detected signals respectively, PA1 a first subtracter for subtracting the second detected signal of the second sub-beam from the first detected signal of the first sub-beam and outputting a first subtracted signal, PA1 a low-pass filter (LPF) for passing low frequency component of the first subtracted signal, where frequencies of the high frequency components are lower than a predetermined value, PA1 a second subtracter for subtracting the second divisional signal from the first electric divisional signal and outputting a second subtracted signal, PA1 a high-pass filter (HPF) for passing high frequency components of the second subtracted signal, where frequencies of the high frequency components are higher than the predetermined value, PA1 an adder for adding outputs of the LPF and HPF together and outputting a tracking error signal, and PA1 a tracking drive circuit for controlling the actuator displacing the objective lens.
4) To be resistant to a change of impinging optical beam, being perpendicular to the disk surface, and a change of optimum focusing of the impinging optical beam spot on the disk surface.
5) To obtain a control servo system which is resistive to scratches on a disk surface.
In a CD player, a 3 beam system is generally utilized.
The 3 beam system is superior on the points of the above terms 3 and 4, thus is utilized in the CD player.
In order to cover the problem of the above term 1, the CD system employs the EFM (Eight to Fourteen Modulation) system in recording the signal, but takes no countermeasure to the above term 2 because the CD format has low recorded signal density and thus the term 2 poses no problem, further, the CD system takes a countermeasure to the above term 5 by employing an improved electric circuit which is effective enough for the low recorded signal density.
Conventional methods of the 3 beam system and a 1 beam push-pull system will be explained hereafter.
FIGS. 5(A) through 5(C) show an operation of 3 beam system. FIG. 6 illustrates 1 beam push-pull system. FIG. 1 shows a block diagram of a conventional tracking servo system. FIG. 2 shows a block diagram of a conventional photodetection and reproduction control circuit.
A conventional optical disk apparatus 100 will be explained referring to FIG. 1. In FIG. 1, 1 is a light source, 2 a diffraction grid, 3 a semi-transparent prism, 4 a collimator lens, 5 an objective lens, 6 a concave lens, 7 a cylindrical lens, 8 a photodetecting device, 9 a reproduction control circuit, 10 a tracking drive circuit, 11 an actuator, D a disk, and D1 a disk surface .
The operation of the conventional optical disk apparatus 100 will be explained as follows referring to FIG. 1.
A light beam projected from the light source 1 passes through the diffraction grid 2, then is incident on the semi-transparent prism 3. The half of the light beam passes through the semi-transparent prism 3 in the direction of the collimator lens 4. The light beam is made to be parallel by the collimator lens 4 and is incident on the objective lens 5. The light beam is focused by the objective lens 5 on a track formed on the disk surface D1 of the disk D. The light beam is reflected by the disk surface D1 of the disk D and back-tracks the objective lens 5 and the collimator lens 4. Then the reflected light beam is incident on the the semi-transparent prism 3 and reflected toward the concave lens 6. The reflected light beam is enlarged to a predetermined magnification by the concave lens 6, passes through the cylindrical lens 7, and irradiates the photodetecting device 8. The photodetecting device 8 converts photoenergy of the incident light beam into electronic energy, and outputs it to the reproduction control circuit 9.
When a 3-beam tracking system is applied to the servo system of FIG. 1, a main beam and 2 sub-beams are produced as the beam generated by the light source through the diffraction grid 2.
A projected image on a disk surface D1 of light beams of the 3 beam system of the disk apparatus 100, guided from the objective lens 5 to the disk surface D1, is shown in FIGS. 5(A) through 5(C). In FIGS. 5(A) to 5(C), S12 is a main beam spot for reproduction of recorded information, and S11 and S13 are sub-beam spots for detecting tracking error of the main beam spot S12. Moreover, FIG. 6 shows a schematic view of the photodetecting device 8 and the reproduction control circuit 9.
In case that the optical disk apparatus 100 employs a 3-beam tracking system, photodetection and reproduction control circuit is shown in FIG. 2, in which the photodetecting device 8 consist of a first and a second photodetectors 8A, 8C respectively, and a 4-division photodetector 8B. The first photodetector 8A receives the sub-beam spot 11 (shown in FIGS. 5(A) to 5(C)) which is reflected from the disk surface D1 and passes through the objective lens 5, the collimator lens 4, the semi-transparent prism 3, the concave lens 6, and the cylindrical lens 7, and outputs an electronic signal therefrom. The 4-division photo detector 8B receives the main beam spot S12 reflected from the disk surface D1, passing through the objective lens 5, the collimator lens 4, the semi-transparent prism 3, the concave lens 6, and the cylindrical lens 7, and outputs 4 separate electric signals therefrom.
Precisely, the 4-division optical detector 8B has 4 divisional photodetectors, 8BA, 8BB, 8BC, and 8BD.
The outputted electric signals of the photodetectors 8A and 8C are amplified by a first and a second amplifiers 9A and 9B respectively, and supplied to a positive terminal and a negative terminal of a comparator 9C respectively, which outputs a difference of the above 2 outputted electric signals as a tracking error signal to the tracking drive circuit 10. The tracking drive circuit 10 drives the actuator 11 which moves the objective lens 5 so as to make the tracking error signal zero.
As mentioned above, the main beam spot S12 is received by the 4 division photodetectors 8BA through 8BD. Two signals outputted respectively from the photodetectors 8BA and 8BC are added each other by an adder 9D. Another pair of signals outputted respectively from the photodetectors 8BD and 8BB are added each other by an adder 9E. Two outputs from the adders 9D and 9E are further added each other by an adder 9F to form an RF signal which is subsequently supplied to an amplifier (not shown). Generally, a reflected beam strength is more weak when it is on a pit of the disk surface D1 than when it isn't.
In FIG. 5(A), if the 3 beams landed are off track to the right side, the strength of the reflected sub-beam spot S13 is weaker than that of the sub-beam spot S11. Then the comparator 9C outputs a tracking error signal according to the difference in strength between the reflected sub-beams S1 and S13. In FIG. 5(B), when the 3 beams irradiate the pit track symmetrically, the strengths of the reflected sub-beam spots S11 and S13 are equal to each other, then the comparator 9C outputs nothing. In FIG. 5(C), when the 3 beams landed are off track to the left side, the strength of the reflected sub-beam spot S13 is stronger than that of the sub-beam spot S11, then the comparator 9C outputs the error signal according to the difference between the strength of the 2 sub-beam spots S11 and S13.
When the optical disk apparatus 100 employs a 1 beam push-pull tracking system, a light beam traces a pit track on a disk surface D1 of disk D as shown in FIG. 6. As the track pitch of the optical disk D is almost equal to the wavelength of the light source 1, such tracks act as a diffraction grating. In FIG. 6, in addition to an original light beam, only first order diffracted components are shown as they are dominant. On a 4 divisional optical detector 8Bb, the original light beam and the first order diffracted components are superimposed each other (shown as hatched areas in FIG. 6), and a tracking error signal is derived by analysing the added value. In this system, the 4 divisional optical detector 8Bb has divisional photodetectors 8BbA through 8BbD similar to the structure of the detector 8B. Added output of the divisional photodetectors 8BbA and 8BbD is supplied to a positive terminal of a comparator 9G, and added output of the divisional photo detector 8BbB and 8BbC is supplied to a negative terminal of the comparator 9G, then the comparator 9G outputs the tracking error signal according to the difference of them, supplying it to the tracking drive circuit 10.
In recent years, optical information recording technology has made a remarkable progress.
High density information recording requires a narrow pitched tracks and small pits. For such high density recording, prior arts had problems as follows.
In the CD recording format, a signal level in a low frequency range is made to be smaller than that in another frequency range (a peak resides between 3T and 11T, wherein T denotes a unit bit period) for reducing low frequency signal components, and this method is called Eight to Fourteen Modulation (EFM). In detail, some bits are added for this purpose, according to the technology called Digital Sum Variation (DSV) control. However, the number of these added bits has to be cut down for increasing a linear recording density. As a result, the level of low frequency signal component will increase, because the low frequency signal components become close to the above peak. For the 3 beam system, the signal of this low frequency signal components behave as a large noise to the tracking error signal, and prevents the optical disk apparatus from its stable tracking and track-access controls.
Hereafter, effects of the low frequency components on a stability of an optical disk apparatus will be explained.
In the CD system, digitized information are recorded in the EFM format on a disk surface forming pits in a form of track, of which a length of pit varies 3T to 1T. In reproduction, by irradiating a laser beam on a track, a pit length and a land length are detected, and digitized information is reproduced as EFM demodulation. FIG. 7(A) shows a frequency power spectrum of reproduced signal.
A signal's frequency component is expressed as a reciprocal of the sum of a pit length and a land length. In the frequency range between 1/11T.times.2 and 1/3T.times.2, the signal is high, and in other range, its level is low, as shown in FIG. 7(A). A frequency range for tracking servo control is indicated in FIG. 7(A), and when a reproduced signal falls into this frequency range, it disturbs the servo control as a noise causing a tracking error.
Such tracking error mechanism will be explained with reference to FIGS. 7(B) and 7(C), in which recorded pits and a land portion are shown. FIG. 7(B) shows beam spots of the 3 beam system, and FIG. 7(C) shows a beam spot of the 1 beam push-pull system. In FIG. 7(B), characters BA through BD, E and F correspond to the photodetectors in FIGS. 2 and 4. In FIG. 7(C), characters BA through BD, correspond to the photodetectors 8BbA to 8BbD in FIG. 6.
Tracking error (TE) signal of the 3 beam system is expressed as Eq.(1), where "E" and "F" represent outputs of the corresponding photodetectors. EQU TE=E-F (1)
Tracking error signal of the 1 beam push pull system is expressed as Eq.(2), where "BA" through "BD" represent outputs of the corresponding photodetectors. EQU TE=(BA+BD)-(BB+BC) (2)
In the 3 beam system, laser beam spots E and F are positioned apart each other, therefore, spots E and F are reflected by different pits respectively, this causes that detected signals of E and F generally contain different pit information each other, thus, a high frequency component of the TE is not cancelled by mixing E and F. Then, the servo system is subject to the high frequency component of these pit information.
In the 1 beam push pull system, the laser beam spot landing on the photodetectors BA and BB, BC and BD is a reflection of a same pit and Eq.(2) teaches that pit information is canceled. This means that high frequency components included in the TE is small, thus, it is much less influencial particularly to the servo system which operates in the low frequency region.
Shortcomings of the tracking control system of the 3 beam system are following.
The 1 beam push-pull system does not have shortcomings mentioned the above 1) through 3), but has other shortcomings as follows.